linux-kernel - Re: [PATCH] scheduler: conditional statement cleanup

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Date:   Sat, 22 Sep 2018 12:26:40 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     PierceGriffiths <pierceagriffiths@...il.com>
Cc:     Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] scheduler: conditional statement cleanup

On Fri, Sep 21, 2018 at 03:22:03PM -0500, PierceGriffiths wrote:
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 625bc9897f62..443a1f235cfd 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -617,12 +617,8 @@ bool sched_can_stop_tick(struct rq *rq)
>  	 * If there are more than one RR tasks, we need the tick to effect the
>  	 * actual RR behaviour.
>  	 */
> -	if (rq->rt.rr_nr_running) {
> -		if (rq->rt.rr_nr_running == 1)
> -			return true;
> -		else
> -			return false;
> -	}
> +	if (rq->rt.rr_nr_running)
> +		return rq->rt.rr_nr_running == 1;
>  
>  	/*
>  	 * If there's no RR tasks, but FIFO tasks, we can skip the tick, no

That one is OK I suppose.

> diff --git a/kernel/sched/cpufreq.c b/kernel/sched/cpufreq.c
> index 5e54cbcae673..a8fd4bd68954 100644
> --- a/kernel/sched/cpufreq.c
> +++ b/kernel/sched/cpufreq.c
> @@ -34,10 +34,7 @@ void cpufreq_add_update_util_hook(int cpu, struct update_util_data *data,
>  			void (*func)(struct update_util_data *data, u64 time,
>  				     unsigned int flags))
>  {
> -	if (WARN_ON(!data || !func))
> -		return;
> -
> -	if (WARN_ON(per_cpu(cpufreq_update_util_data, cpu)))
> +	if (WARN_ON(!data || !func || per_cpu(cpufreq_update_util_data, cpu)))
>  		return;
>  
>  	data->func = func;

But I'm not a fan of this one. It mixes a different class of function
and the WARN condition gets too complicated. Its easier to have separate
warns.

> diff --git a/kernel/sched/cpupri.c b/kernel/sched/cpupri.c
> index daaadf939ccb..152c133e8247 100644
> --- a/kernel/sched/cpupri.c
> +++ b/kernel/sched/cpupri.c
> @@ -29,20 +29,16 @@
>  #include "sched.h"
>  
>  /* Convert between a 140 based task->prio, and our 102 based cpupri */
> -static int convert_prio(int prio)
> +static int convert_prio(const int prio)
>  {
> -	int cpupri;
> -
>  	if (prio == CPUPRI_INVALID)
> -		cpupri = CPUPRI_INVALID;
> +		return CPUPRI_INVALID;
>  	else if (prio == MAX_PRIO)
> -		cpupri = CPUPRI_IDLE;
> +		return CPUPRI_IDLE;
>  	else if (prio >= MAX_RT_PRIO)
> -		cpupri = CPUPRI_NORMAL;
> +		return CPUPRI_NORMAL;
>  	else
> -		cpupri = MAX_RT_PRIO - prio + 1;
> -
> -	return cpupri;
> +		return MAX_RT_PRIO - prio + 1;

The code looks even better if you leave out the last else.

>  }
>  
>  /**
> @@ -95,10 +91,8 @@ int cpupri_find(struct cpupri *cp, struct task_struct *p,
>  		smp_rmb();
>  
>  		/* Need to do the rmb for every iteration */
> -		if (skip)
> -			continue;
> -
> -		if (cpumask_any_and(&p->cpus_allowed, vec->mask) >= nr_cpu_ids)
> +		if (skip || cpumask_any_and(&p->cpus_allowed, vec->mask)
> +				>= nr_cpu_ids)
>  			continue;
>  
>  		if (lowest_mask) {

That just makes the code ugly for no reason.

> @@ -222,7 +216,7 @@ int cpupri_init(struct cpupri *cp)
>  	return 0;
>  
>  cleanup:
> -	for (i--; i >= 0; i--)
> +	while (--i >= 0)
>  		free_cpumask_var(cp->pri_to_cpu[i].mask);
>  	return -ENOMEM;
>  }
> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> index 2e2955a8cf8f..acf1b94669ad 100644
> --- a/kernel/sched/rt.c
> +++ b/kernel/sched/rt.c
> @@ -142,10 +142,12 @@ void free_rt_sched_group(struct task_group *tg)
>  		destroy_rt_bandwidth(&tg->rt_bandwidth);
>  
>  	for_each_possible_cpu(i) {
> -		if (tg->rt_rq)
> -			kfree(tg->rt_rq[i]);
> -		if (tg->rt_se)
> -			kfree(tg->rt_se[i]);
> +		/* Don't need to check if tg->rt_rq[i]
> +		 * or tg->rt_se[i] are NULL, since kfree(NULL)
> +		 * simply performs no operation
> +		 */

That's an invalid comment style.

> +		kfree(tg->rt_rq[i]);
> +		kfree(tg->rt_se[i]);
>  	}
>  
>  	kfree(tg->rt_rq);
> @@ -1015,10 +1017,7 @@ enqueue_top_rt_rq(struct rt_rq *rt_rq)
>  
>  	BUG_ON(&rq->rt != rt_rq);
>  
> -	if (rt_rq->rt_queued)
> -		return;
> -
> -	if (rt_rq_throttled(rt_rq))
> +	if (rt_rq->rt_queued || rt_rq_throttled(rt_rq))
>  		return;
>  
>  	if (rt_rq->rt_nr_running) {

The compiler can do this transformation and the old code was simpler.

> @@ -1211,10 +1210,7 @@ void dec_rt_tasks(struct sched_rt_entity *rt_se, struct rt_rq *rt_rq)
>   */
>  static inline bool move_entity(unsigned int flags)
>  {
> -	if ((flags & (DEQUEUE_SAVE | DEQUEUE_MOVE)) == DEQUEUE_SAVE)
> -		return false;
> -
> -	return true;
> +	return !((flags & (DEQUEUE_SAVE | DEQUEUE_MOVE)) == DEQUEUE_SAVE)
>  }

Again, I find the new code harder to read.

>  
> @@ -2518,12 +2513,10 @@ static int tg_set_rt_bandwidth(struct task_group *tg,
>  	/*
>  	 * Disallowing the root group RT runtime is BAD, it would disallow the
>  	 * kernel creating (and or operating) RT threads.
> +	 *
> +	 * No period doesn't make any sense.
>  	 */
> -	if (tg == &root_task_group && rt_runtime == 0)
> -		return -EINVAL;
> -
> -	/* No period doesn't make any sense. */
> -	if (rt_period == 0)
> +	if ((tg == &root_task_group && !rt_runtime) || !rt_period)
>  		return -EINVAL;
>  
>  	mutex_lock(&rt_constraints_mutex);

Again, far harder to read.

In short, while all the transformations are 'correct' the end result is
horrible. Please don't do this.